Molecular sieve materials, both natural and synthetic, have been demonstrated in the past to be useful as adsorbents and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain molecular sieves, such as zeolites, AIPOs, and mesoporous materials, are ordered, porous crystalline materials having a definite crystalline structure as determined by X-ray diffraction (XRD). Within the crystalline molecular sieve material there are a large number of cavities which may be interconnected by a number of channels or pores. These cavities and pores are uniform in size within a specific molecular sieve material. Because the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of industrial processes. The zeolite pores may be in the micro- (<2 nm), meso- (2 to 50 nm) or macro (>50 nm to 200 nm) size range.
Such molecular sieves, both natural and synthetic, include a wide variety of crystalline silicates. These silicates can be described as rigid three-dimensional frameworks of SiO4 tetrahedra (which have four oxygen atoms at the apexes with the silicon atom being at the center) and Periodic Table Group 13 element oxide (e.g., AlO4, BO4) tetrahedral (which have four oxygen atoms at the apexes with the Periodic Table Group 13 element being at the center). These tetrahedra are regularly and three dimensionally cross-linked by the sharing of oxygen atoms. This arrangement provides a three-dimensional network structure defining pores that differ in size and shape, depending on the arrangement of tetrahedral and composition of the structure. The electrovalence of the tetrahedra containing the Group 13 element (e.g., aluminum or boron) is balanced by the inclusion in the crystal of a cation, for example a proton, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group 13 element (e.g., aluminum or boron) to the number of various cations, such as H+, Ca2+/2, Sr2+/2, Na+, K+, or Li+, is equal to unity. It is the presence of framework aluminum in aluminosilicates which is important in providing, for instance, the catalytic properties of these materials.
Molecular sieves that find application in catalysis include any of the naturally occurring or synthetic crystalline molecular sieves. Examples of these molecular sieves include large pore zeolites, intermediate pore size zeolites, and small pore zeolites. These zeolites and their isotypes are described in “Atlas of Zeolite Framework Types”, eds. Ch. Baerlocher, L. B. McCusker, D. H. Olson, Elsevier, Sixth Revised Edition, 2007, which is hereby incorporated by reference.
Synthesis of molecular sieve materials typically involves the preparation of a synthesis mixture which comprises sources of all the elements present in the molecular sieve often with a source of hydroxide ion to adjust the pH. In many cases a structure directing agent is also present. Structure directing agents are compounds which are believed to promote the formation of molecular sieves and which are thought to act as templates around which certain molecular sieve structures can form and which thereby promote the formation of the desired molecular sieve. Various compounds have been used as structure directing agents including various types of quaternary ammonium cations.
The synthesis of molecular sieves is a complicated process. There are a number of variables that need to be controlled in order to optimise the synthesis in terms of purity, yield and quality of the molecular sieve produced. A particularly important variable is the choice of synthesis template (structure directing agent), which usually determines which framework type is obtained from the synthesis. Quaternary ammonium ions are typically used as the structure directing agents in the preparation of zeolite catalysts.
The “as-synthesised” molecular sieve will contain the structure directing agent in its pores, and is usually subjected to a calcination step to burn out the structure directing agent and free up the pores. For many catalytic applications, it is desired to convert the molecular sieve to the hydrogen form (H-form). That may be accomplished by firstly removing the structure directing agent by calcination in air or nitrogen, then ion exchanging to replace alkali metal cations (typically sodium cations) by ammonium cations, and then subjecting the molecular sieve to a final calcination to convert the ammonium form to the H-form. The H-form may then be subjected to various ‘post-treatments” such as steaming and/or acid treatments to remove aluminum or other metal ions from the framework. The products of such treatments are often referred to as “post-treated”.
Mordenite, a member of the large-pore zeolite family, consists of 12-membered ring pore channels interconnected by 8-membered ring pores. However, the 8-membered ring pores are too small for most molecules to enter, and so mordenite is generally considered a one-dimensional pore system. Despite this feature, mordenite is widely used in industry, particularly for alkylation, transalkylation, and (hydro) isomerization reactions. To improve physical transport in the 1-D channels, mordenite crystals are typically subjected to dealumination post-treatment. Post-treated mordenite catalysts have been used for transalkylation of heavy aromatics and have shown very encouraging performance. Mordenite is commercially available from, for example, Tosoh and Zeolyst. There is a desire to provide improved mordenite catalysts having improved catalytic performance.